Overview

Interharmonics—signals at frequencies that are not integer multiples of 60 Hz—are an increasingly common by-product of modern power electronics and renewable generation. Unlike traditional harmonics, they do not cancel cleanly in RMS calculations, and their presence can cause RMS values to fluctuate noticeably from cycle to cycle.

This white paper explores the mathematical mechanism behind that RMS variation, showing how interharmonic content introduces low-frequency modulation that may not be obvious from waveform shape alone. Even small interharmonic components can lead to perceptible voltage flicker or anomalous readings—especially when superimposed on distorted waveforms.

Key topics include:

• What differentiates interharmonics from harmonics
• Why RMS values become time-varying in their presence
• How specific frequency combinations create visible flicker
• A worked example showing how 175 Hz content causes 5 Hz RMS modulation

For utility engineers and power quality professionals, understanding interharmonic behavior is essential for diagnosing RMS anomalies and customer complaints related to flicker. This paper provides the theoretical foundation and practical context to recognize when interharmonics are the culprit—and why they matter in modern distribution systems.